The invention relates to a process for the preparation of a two-coat finish on a substrate surface, in which
(1) a pigmented aqueous basecoat is applied to the substrate surface, PA0 (2) a polymer film is formed from the basecoat applied in step (1), PA0 (3) a transparent topcoat is applied to the basecoat obtained in this way, and subsequently PA0 (4) the basecoat and the topcoat are baked together.
The invention also relates to aqueous coating materials which can be employed in this process as pigmented basecoats.
The process described above is employed in particular for the preparation of two-coat automotive finishes of the basecoat/clearcoat type. In this context the quality of the two-coat finish prepared by this process depends quite critically on the aqueous basecoat employed in step (1) of the process.
EP-A-353 797 describes aqueous coating materials which can be employed as basecoats in step (1) of the above-described process. The aqueous coating materials described in EP-A-353 797 contain as binder a polymer which is obtainable by subjecting acrylate and/or methacrylate monomers to an emulsion polymerization which is initiated by water-soluble initiators, in the presence of an anionic polyurethane resin which may also contain vinyl groups.
If the aqueous coating materials described in EP-A-353 797 are employed as basecoats in the above-described process, two-coat finishes are obtained which have an unsatisfactory stability with respect to condensed moisture. This disadvantage manifests itself in particular in refinishes, which are only cured at temperatures up to 80.degree. C. Moreover, the aqueous coating materials described in EP-A-353 797 have an unsatisfactory storage stability if they contain a melamine resin as additional binder component.
EP-A-297 576 describes a process for the preparation of two-coat finishes of the type described above, in which the aqueous coating materials employed as basecoat contain an aqueous polymer dispersion which is obtainable by polymerizing ethylenically unsaturated monomers in an aqueous dispersion in the presence of a polyurethane resin which contains urea groups but no vinyl groups. If the aqueous coating materials described in EP-A-297 576 are employed as basecoats in the above-described process for the preparation of two-coat finishes, then the two-coat finishes obtained are in need of improvement with respect to their resistance to condensed water. Moreover, the aqueous coating materials described in EP-A-297 576 often display deficiencies in storage stability, and defects which can be traced back to instances of incompatibility, if combinations of different binders are employed.
DE-A-40 10 176 describes a process for the preparation of a two-coat finish, in which an aqueous basecoat is employed which contains as binder a polymer which is obtainable by polymerizing ethylenically unsaturated monomers in an organic solvent in the presence of a polyurethane resin which contains polymerizable double bonds and converting the resulting reaction product to an aqueous dispersion. If basecoats which contain metal pigments are employed in the process described in DE-A-40 10 176, the two-coat metallic finishes obtained are in need of improvement with respect to their metallic effect. Moreover, a disadvantage in the preparation of the aqueous basecoats described in DE-A-40 10 176 is that that a large quantity of organic solvents is required.
The object of the present invention was to provide a new process for the preparation of two-coat finishes of the type described above, with which two-coat finishes are obtained whose properties are improved in comparison with the prior art and in which, in particular, the above-described disadvantages of the prior art are absent or lessened.
This object is surprisingly achieved by a process for the preparation of a two-coat finish on a substrate surface, which process consists in going through the steps (1) to (4) described above and is characterized in that the basecoat contains as binder a polymer which can be obtained by subjecting an ethylenically unsaturated monomer or a mixture of ethylenically unsaturated monomers to free-radical polymerization in an aqueous dispersion of a polyurethane resin which has a number-average molecular weight of from 1000 to 30,000 and contains on average from 0.05 to 1.1 polymerizable double bonds per molecule, and in the presence of a water-insoluble initiator or of a mixture of water-insoluble initiators, the weight ratio of the polyurethane resin to the ethylenically unsaturated monomer or to the mixture of ethylenically unsaturated monomers being between 1:10 and 10:1.
In cases where a basecoat is employed which contains metal pigment the two-coat finishes prepared by the process according to the invention exhibit an excellent metallic effect. The adhesion between basecoat and substrate and the adhesion between basecoat and clearcoat is excellent. Moreover, the two-coat finishes prepared by the process according to the invention exhibit a very good resistance to high atmospheric humidity. The aqueous basecoats employed in accordance with the invention are stable on storage and exhibit no defects which can be traced back to incompatibility phenomena, even when combinations of different binders are employed.